The present invention is directed to electric filters, and more particularly to multi-resonator electric filters.
Electrical filters are generally known and often include electrical components, such as inductors, capacitors, and resistors. Filters are often used to select desired electric signal frequencies that will be passed through the filter while blocking or attenuating other undesirable electric signal frequencies. Filters may be classified in some general categories that include low-pass filters, high-pass filters, band-pass filters, and band-stop filters, indicative of the type of frequencies which are selectively passed by the filter. Further, filters can be classified by type, such as Butterworth, Chebyshev, Inverse Chebyshev, and Elliptic, indicative of the type of bandshape response (frequency cutoff characteristics) the filter provides relative to the ideal.
Further, the filters often include capacitors and inductors in series or parallel and may include multiple stages or poles that may be resonators. For example, a capacitor and inductor set may make up a resonator, and a four-pole filter may include four resonators each having a capacitor (C) and inductor (L) set. For example, a circuit schematic for an eight-pole band-pass filter is provided in FIG. 1. In this case, each L and C pair are resonators and each of the resonators are capacitively coupled to one another in series. The first resonator 101 includes two capacitors, C1 and C2, and an inductor L1. There are eight such resonators 101-108 making up the eight-pole band-pass filter.
Filters are often used in communication systems. For example, one particular application is for cellular communications and includes the formation of filters useful in the microwave range, such as frequencies above 500 MHz, for base-station transceivers.
Considering the case of conventional microwave filters, there have been basically four types. First, lumped-element filters have used separately fabricated air wound inductors and parallel-plate capacitors, wired together into a filter circuit. These conventional components are relatively small compared to the wave length, and accordingly, make for a fairly compact filter. However, the use of separate elements has proved to be difficult in manufacture, and resulting in large circuit to circuit differences. The second conventional filter structure utilizes mechanical distributed element components. Coupled bars or rods are used to form transmission line networks that are arranged as a filter circuit. Ordinarily, the length of the bars or rods is xc2xc or xc2xd of the wave length at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. The structures also suffer from various responses at multiples of the center frequency. Fourth, cavity filters have been used. They also suffer from various responses at multiples of the center frequency and can be quite large.
Various thin-film lumped-element structures have been proposed. Swanson U.S. Pat. No. 4,881,050, issued Nov. 14, 1989, discloses a thin-film microwave filter utilizing lumped elements. In particular, a capacitor xcfx80 network utilizing spiral inductors and capacitors is disclosed. Generally, a multi-layer structure is utilized, a dielectric substrate having a ground plane on one side of the substrate and multiple thin-film metal layers and insulators on the other side. Filters are formed by configuring the metal and insulation layers to form capacitive xcfx80-networks and spiral inductors. Swanson U.S. Pat. No. 5,175,518 entitled xe2x80x9cWide Percentage Band With Microwave Filter Network and Method of Manufacturing Samexe2x80x9d discloses a lumped-element thin-film based structure. Specifically, an alumina substrate has a ground plane on one side and multiple layer plate-like structures on the other side. A silicon nitride dielectric layer is deposited over the first plate on the substrate, and a second and third capacitor plates are deposited on the dielectric over the first plate.
Historically, such lumped element circuits were fabricated using normal, that is, non-superconducting materials. These materials have an inherent loss and, as a result, the circuits have various degree of lossiness. For resonant circuits, the loss is particularly critical. The Q of a device (assumed to be xe2x80x9cunloadedxe2x80x9d throughout this document) is a measure of its ability to store energy and thus inversely related to its power dissipation or lossiness. Resonant circuits fabricated from printed normal metals have Q""s at best on the order of a few hundred.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from these materials. The microwave properties of the high temperature superconductors have improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. B. Hammond, et al., xe2x80x9cEpitaxial Tl2Ca1,Ba2Cu2O8Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77 Kxe2x80x9d, Appl. Phys. Lett., Vol. 57, pp. 825-27, 1990. Various filter structures and resonators have been formed. Other discrete circuits for filters in the microwave region have been described. See, e.g., S. H. Talisa, et al., xe2x80x9cLow-and High-Temperature Superconducting Microwave Filters,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September 1991, pp. 1448-1554.
The need for compact, reliable narrow-band filters has never been stronger. Applications in the telecommunication fields are of particular importance. As more users desire to use the microwave band, the use of more narrow-band filters helps to increase the number of users in the spectrum. The area from 700 to 2,000 MHz is of particular interest. In the United States, the 800 to 900 MHz range is used for analog and digital cellular communications. The personal communications services (PCS) are in the 1,800 to 2,000 MHz range.
Many passive microwave devices, for example, resonators, filters, antennas, delay lines, and inductors, have been fabricated in planar form utilizing high temperature superconducting thin films. As described, such structures are often smaller than conventional technologies in terms of physical size. However, these devices are also limited in their size given the constraints of fabricating high quality, epitaxial films. As a result, devices fabricated in HTS films are often of a quasi-lumped element nature, that is, where the nominal size the device is smaller than the wavelength of operation. This often results in folding of devices, which leads to significant coupling between lines.
Despite the clear desirability of improved electrical circuits, including the known desirability of converting circuitry to include superconducting elements, efforts to date have not always been satisfactory. It has proved to be difficult in substituting high temperature superconducting materials to form circuits without degrading the intrinsic Q of the superconducting film. These problems include circuit structure, radiative loss and tuning, and have remained in spite of the clear desirability of an improved circuit. Some of these problems have been overcome by the inventions discloses in U.S. patent application Ser. Nos. 5,888,942 and 6,026,311. However, there is still room for further improvements of relatively high Q and reduced intermodulation distortion (IMD) of electric filters in general. This need is particularly applicable to superconducting electric filters used in, for example, wireless telecommunication systems such as cellular communications base-station and mobile-station transceivers.
While relatively only small losses occur in many superconducting filters, superconducting filters are inherently nonlinear systems. Filter nonlinearities can limit the intermodulation intercept point of, for example, a base-station receiver to values that are too small for certain demanding applications. For example, sometimes conventional superconducting filters cannot be effectively used in wireless telecommunication networks where the base stations are co-located with strong specialized mobile radio (SMR) transmitters or with other cellular/PCS service providers because the power levels of out-of-band signals from these other systems can be too high and can result in IMD that reduces the receiver sensitivity. As a result, the superconducting filters are unable to adequately filter out the undesired out-of-band signals. The performance of the filter also changes with manufacturing process variations of the resonators and filters. Although some filters might be manufactured to achieve the required filtering capabilities for filtering out competing system out-of-band signaling, many of them would fail in such applications and are thus sorted out during testing, resulting in low filter manufacturing yields. Therefore, there is a need to improve electric filters design so that they operate with reduced IMD, and result in increased manufacturing yield.
The present invention is directed to electric filters with improved intermodulation distortion characteristics and a method for designing such electric filters. In general, the invention includes a multiple stage or pole (e.g., multi-resonator) electric filters in which one or more of the stages have been intentionally designed to have different electrical performance characteristics (e.g., signal filter performance) than the other resonators in the electric filter. In one case, the electric filters include multiple resonators coupled together with at least two of the multiple resonators having an intermodulation intercept point (IP) and/or Q different from one another. The relative Q and IP of the respective resonators may be determined by the relative strength of in-band and out-of-band signals expected in the application. The performance and cost of the electric filter may be optimized by designing the filter to have a relative Q and IP required by the particular application.
In one embodiment, the electric filter is a multi-resonator superconducting filter useful in, for example, wireless communication systems. The design of the filter assembly is determined by identifying those critical resonators that have the greatest impact on intermodulation distortions and losses and altering those critical resonators to minimize the intermodulation-distortion products while still maximizing Q. The superconducting filter may be, for example, a multi-resonator Chebyshev band-pass filter in which the first, and possibly the last, resonators have a different nth-order intercept point (IPn) and/or Q. For example, the intermodulation intercept point of the filter can be increased by many orders of magnitude by increasing the IPn of the first resonator of the multi-resonator Chebyshev band-pass filter assembly. The first resonator may have lower Q relative to the other resonators, if the filter IP can be made higher with minimal degradation of the overall filter Q. Further, the last resonator may have low Q and low IPn. All other resonators may have high Q and high IPn. This combination of resonators is most advantageous for situations where the out-of-band signals are strong and the in-band signals are moderately strong to strong. In one variation the multiple resonators may be coupled in series and each resonator may comprise a set of capacitors and an inductor. Using this design approach a multi-resonator filter may be created which has reduced IMD with relatively high Q on average.
In another embodiment, the filter may be designed for situations in which the out-of-band signals are strong and the in-band signals are weak. In this case, the filter may have the best performance and cost with a high IP if the Q is low and the IPn is high for the first resonator of the multi-resonator Chebyshev band-pass filter assembly. Further, the last resonator may have low Q and low IPn while all other resonators may have high Q and low IPn.
In a still further embodiment, the filter may be designed for situations in which the out-of-band signals are moderately strong and the in-band signals are moderately strong. In this case, the filter may have the best performance and a high IP if the Q is low and the IPn is high for the first resonator of the multi-resonator Chebyshev band-pass filter assembly. Further, the last resonator may have low Q and low IPn while all other resonators may have high Q and high IPn.
In an even further embodiment, the filter may be designed for situations in which the out-of-band signals are weak to moderately strong and the in-band signals are weak. In this case, the filter may have the best performance and cost with a high IP if the Q is low and the IPn is low for the first resonator of the multi-resonator Chebyshev band-pass filter assembly. Further, the last resonator may have low Q and low IPn while all other resonators may have high Q and low IPn.
The approach taught by the present invention for designing multi-stage filters may be most powerful in applications in which only a few resonators compromising the filter could be changed because of physical size limitations of the filter in the application. Further, this design approach may be used to enable use of new resonator designs that have superior properties when used in a small number of poles (e.g., 2-3 poles) but which would lead to unfeasible features when many of them are used, as in higher order filters (e.g., 4 or more poles). The design approach of the present invention may also be beneficial when only resonators with given, although different, electrical performance characteristics are available. For example, some resonators having low Q and a low IPn might still be used in the filter assembly. As such, the design approach of the present invention may specify how each of the various stages in a filter should be designed or assembled using, for example, particular individual resonators having particular electrical performance properties, so that (1) the filter performance may be improved, (2) the variability in the manufacturing process may be reduced, and (3) the yield of the manufacturing process may be increased. The invention, although explained using superconducting filters, applies equally well to any filter structures that are nonlinear and/or lossy.